I found this article (http://autos.yahoo.com/maintain/repairqa/engine/ques057_1.html) to be very enlightening. When AirRandy had very poor compression in one of his cylinders and decided to pull the head. Upon removing the head and looking at it, he was pretty sure the low compression was related to what we were seeing on the top of the engine. As an additional check put a bit of diesel into the cylinders and watched to see if it flowed through quickly or slowly. If it went down quickly it would have indicated there was also potentially a problem with the cylinders, pistons, rings, etc.

Are you having engine issues or just thinking about doing the test to see what it tells you about your engine?

subzali

09-28-2009, 01:22 PM

Cool, thanks Ricardo - that helps. So what kind of tools are needed to perform such a test (i.e. do they make standard tools that thread into the head, have a quick-disconnect at the other end and a pressure gauge in the middle)? And how do you test % leakage? How long do you wait, or is it fairly instantaneous?

DaveInDenver

09-28-2009, 01:34 PM

Special leak down tester is used. You put pressure on the cylinder from your air compressor and see how much leaks when you lock the piston at TDC in the bore. You're looking for bad valve seal, ring issues, etc. The spinning compression check shows compression itself, but does not tell you much about why the compression would be low. The leak down will indicate where the problem is, for example if your dip stick shoots out then you probably have ring seal problems, if air rushes through the carb or tail pipe, valves are not sitting down or you have the valve lash too tight or cam timing off, if the pressure tester flies back at you the plug threads are stripped, if the coolant blows out of the radiator the HG is probably bad, etc.

It happens quickly, no waiting. It's a little different than a cooling system leak down where you pressurize the system and leave it overnight to see how much it has gone down. The percentage is a ratio of the input pressure again what the cylinder is holding. So say you put 100 psi on the tester and the bore pressure is 95 psi, you have 5% leakage. The tester will either have one gage or two. The nicer one is two gages with adjustment, but a single gage works if you know the pressure at the end of your hose (which as you know is different than the reading on the tank).

A few percent is pretty normal, even a brand new engine will have a few percent leakage since the chamber pressure is what pushes the rings against the cylinder wall. You have to worry when the leakage is 20~25%. This has nothing to do with the ratio. You might have awesome or crappy compression ratios and still have a tight engine. This is the case with my mighty 22R since I did not shave the head enough to compensate for the cam and combustion chamber work, so I have lower compression than I expected but very little leakage.

SteveH

09-28-2009, 02:28 PM

Re: special tools: If you have a compression tester with a detachable hose (as sold by Sears and others), you're halfway there. With this, you can pressurize the cylinders.

If you remove the Schraeder valve insert in the hose, and you'll have a hose that threads into your cylinder on one end, and attaches to your air compressor on the other end. You can certainly pressurize the cylinders (one by one, rotating the engine between each) and listen for air rushing out of the carb, exhaust or crankcase. This won't give you numeric values, but it would point out a valve that is failing to seat and similar conditions. I've had carbon stick in valves during compression tests, so double-test any cylinder that seems suspect.

nakman

09-28-2009, 02:38 PM

...The leak down will indicate where the problem is, for example if your dip stick shoots out then you probably have ring seal problems, if air rushes through the carb or tail pipe, valves are not sitting down or you have the valve lash too tight or cam timing off, if the pressure tester flies back at you the plug threads are stripped, if the coolant blows out of the radiator the HG is probably bad, etc.
.

OMG lol Dave, :lmao: :beer2:

TIMZTOY

09-28-2009, 03:33 PM

unless your wanting to do a profesionall diag you really don't need the gauges
Most parts stores sell the cheapo hose adaptor that threads I to your sparkplug hole. It will tell you what your needing to know. And is only a few bucks. It about 8" long has twin threads with o-rings on one side for the spark plug threads and smaller threads on the other side for your air hose fitting.

rover67

09-28-2009, 04:25 PM

Some little school I went to taught us to do leak downs at every valve adjustment.

It's easy to do and you might as well if you have the tool. Just be sure the truck has the parking break set, cuz it'll kinda take off when you pressurize the cylinder and it falls off of TDC.

mine had a regulator on it, but using the ones that don't is easy enouhg, just set your shop air to 100psi.